The disclosure of Japanese Patent Application No. 2000-395477 filed on Dec. 2, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an air-fuel ratio control system for an internal combustion engine and a control method thereof.
2. Description of Related Art
In internal combustion engines, an exhaust emission purification catalyst (three-way catalyst) for purifying an exhaust gas and an air-fuel ratio sensor for detecting an air-fuel ratio are arranged in an exhaust passage. A feedback control is performed on the basis of the air-fuel ratio detected by the air-fuel ratio sensor such that the air-fuel ratio of an air-fuel mixture becomes a stoichiometric air-fuel ratio, thereby reducing emissions of nitrogen oxides (NOx), carbon monoxides (CO), and hydrocarbons (HC) at the same time.
Performing the above-mentioned feedback control with a sufficient accuracy effectively improves a purification rate of the exhaust gas emitted by the internal combustion engines. Also, controlling an oxygen adsorption function of the exhaust emission purification catalyst effectively improves the purification rate of NOx, CO, and HC.
Investigations have been conducted on a control for effectively utilizing an oxygen adsorption function. For example, Japanese Patent Application laid-open No. 5-195842 discloses a type of control system which controls the oxygen adsorption function. The control system estimates an amount of oxygen that can be adsorbed in a whole part of the exhaust emission purification catalyst (oxygen storage amount), and controls the air-fuel ratio such that the oxygen storage of an amount of oxygen becomes a certain targeted value.
The above-mentioned control system performs the air-fuel ratio control based on the oxygen storage amount estimated on the assumption that the status of the entire exhaust emission purification catalyst is uniform. However, the oxygen adsorption status in the exhaust emission purification catalyst is not uniform. Hence, in a case where the air-fuel ratio control is performed on the assumption that the oxygen absorption status in the exhaust emission purification catalysis is uniform, there is a possibility that an estimation accuracy will temporarily decrease, and that the air-fuel ratio control will become inaccurate. This creates a drawback such that an excess amount of oxygen storage needs to be secured, and that the oxygen adsorption capacity cannot be efficiently used.
It is an aspect of the invention to improve a purification efficiency of an exhaust gas by effectively utilizing an oxygen adsorption capacity of a catalyst.
According to a first aspect of the invention, an air-fuel ratio control system for an internal combustion engine includes a controller having a calculator which estimates an oxygen storage amount of a catalyst provided in an exhaust passage of an internal combustion engine. The controller controls an air-fuel ratio based on the estimated oxygen amount. The calculator divides the catalyst into multiple sections in a flow direction of an exhaust gas, and calculates a change in the oxygen storage amount in a specified section among the multiple sections based on an air-fuel ratio of the exhaust gas flowing into the catalyst. The controller estimates the oxygen storage amount in the specified section based on a record of the change in the oxygen storage amount. The controller controls the air-fuel ratio based on the oxygen storage amount in the specified section estimated by the calculator.
Further, another aspect of the invention is to provide an air-fuel ratio control method for an internal combustion engine including the steps of dividing the catalyst into multiple sections in a flow direction of an exhaust gas, calculating a change in the oxygen storage amount in a specified section among the multiple sections based on an air-fuel ratio of the exhaust gas flowing into the catalyst, estimating the oxygen storage amount in the specified section based on a record of the change in the oxygen storage amount, and controlling the air-fuel ratio based on the estimated oxygen storage amount in the specified section.